use older glibc

.github/workflows/build.yaml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   rebuild:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-20.04
     permissions:
       packages: write
     steps:
@@ -31,5 +31,5 @@ jobs:
     - name: Push packages and update index
       run: |
         spack mirror set --push --oci-username ${{ github.actor }} --oci-password "${{ secrets.GITHUB_TOKEN }}" github-actions-buildcache
-        spack -e . buildcache push -j $(($(nproc) + 1)) --base-image ubuntu:22.04 --update-index github-actions-buildcache
+        spack -e . buildcache push -j $(($(nproc) + 1)) --base-image ubuntu:20.04 --update-index github-actions-buildcache
       if: ${{ !cancelled() }}

.github/workflows/update-index.yaml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   update-index:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-20.04
     permissions:
       packages: write
     steps:

README.md

@@ -4,8 +4,15 @@ This repo provides a buildcache to speed up Spack in your GitHub Actions.
 
 Currently it provides binaries from Spack `develop` for
 
-- `%gcc@11 os=ubuntu22.04 target=x86_64_v3`
-- `%clang@15 os=ubuntu22.04 target=x86_64_v3`
+- `%gcc@11 ^glibc@2.31 target=x86_64_v3`
+- `%clang@12 ^glibc@2.31 target=x86_64_v3`
+
+which supports the following distros:
+
+- Ubuntu 20.04 and later
+- Debian 11 and later
+- RHEL 9 and later
+- Fedora 32 and later
 
 To use it, add an environment `spack.yaml` to the root of your own repository
 
@@ -21,7 +28,7 @@ spack:
 
   packages:
     all:
-      require: '%gcc@12 target=x86_64_v3'
+      require: 'target=x86_64_v3'
 ```
 
 and Spack install it in a GitHub Action:

spack.yaml

@@ -18,18 +18,18 @@ spack:
       - openmpi
       - perl
       - python@3.11
-    - - '%gcc@11'
-      - '%clang@15'
+    - - '%gcc@9'
+      - '%clang@12'
   - matrix:
     - - fftw +mpi +openmp ^mpich
       - fftw +mpi +openmp ^openmpi
       - openblas threads=openmp +ilp64
-    - - '%gcc@11'
+    - - '%gcc@9'
   # clang does not have openmp libs in GitHub Actions?
   - matrix:
     - - fftw +mpi ^mpich
       - fftw +mpi ^openmpi
-    - - '%clang@15'
+    - - '%clang@12'
   concretizer:
     unify: false
   config: